This invention relates to an optical down-converter for the mapping of received radio frequencies into an arbitrary intermediate frequency range while precluding interference between the received signals. More particularly, the invention relates to a device for improving image rejection to improve the harmonic spurs limiting system performance.
Prior art image rejection systems providing significant frequency translation include those that use a digital phase modulator to produce a serrodyne phase modulated waveform, and those that split a received signal into two parts and then recombine them in such a way as to eliminate the unwanted image and carrier frequencies. Optical image rejection mixers that utilize the serrodyne method of frequency translation apply a sawtooth waveform to phase modulate the optical signal, thereby causing the optical frequency to shift. The achievable image rejection is limited by the number of discrete bits that can be implemented by the digital phase modulator when approximating the sawtooth waveform. This limits the image rejection to approximately 25 dB.
In the case of microwave image rejection mixers, the achievable image rejection is limited by the need for near perfect amplitude and phase control. As shown in FIG. 1, a received radio frequency (RF) signal 124 is first divided into two signals 116 and 118 in a in-phase power divider 122, mixed in associated mixers 112 and 114 with a local oscillator input 102 (LO IN) and split into associated signals 106 and 108 that are shifted 90xc2x0 in a first 3 dB 90xc2x0 hybrid power divider 123. The output of the mixers 112 and 114 are then recombined with a 90xc2x0 phase shift between the two components in a second 3 dB 90xc2x0 hybrid power divider 126 and output as an intermediate frequency band 134. Exact amplitude and phase matching with broad band signals is nearly impossible with strictly microwave components since the frequency response of each component varies. A typical device, with a 3xc2x0 phase error and a 0.25 dB amplitude imbalance upon recombination, is limited to about 36 dB of image and carrier rejection. Further details on serrodyne frequency shifting can be found in Johnson et al., SERRODYNE OPTICAL FREQUENCY TRANSLATION WITH HIGH SIDEBAND SUPPRESSION, J. of Lightwave Tech., Vol. 6, No. 6, pg. 109, 1988.
Another optical image rejection down-converter described in U.S. patent application Ser. No. 09/620,324 by Ward et al., entitled IMAGE REJECTING MICROWAVE PHOTONIC DOWNCONVERTER, Navy Case No. 79,800, filed Jul. 17, 2000, employs an electronic mixer for up-converting signals into the passband of a bandpass filter followed by optical down-converting of the filtered signals into a desired output band, as shown in FIG. 2, thereby providing greater than 60 dB of image rejection. The device allows telecommunications systems to down-convert the lower sideband of densely multiplexed ultra-wideband bandwidth channels into low frequency bands where conventional electronics can perform signal processing function; has the image rejection ( greater than 60 dB) to provide unambiguous signals for direction finding applications; and exhibits efficient image rejection that should permit multi-octave microwave frequency reception and compression. In addition, that invention is intrinsically remoteable, due to the various optical and electrical components that may be used to construct the subject device. The device, however, utilizes a first local oscillator at frequencies below the original frequency of a bandpass filter that may allow harmonics from the local oscillator to convert undesired input radio frequency (RF) frequencies into spurious signals presenting the output intermediate frequency band being applied to user electronics. These harmonics can be handled by the user electronics but additional signal processing would be required.
A recently developed all-optical image rejection system down-converter capable of removing the harmonics before entering the user electronics is described in U.S. patent application Ser. No. 09/635,985 by Strutz et al., entitled ALL OPTICAL IMAGE REJECT DOWN CONVERTER, Navy Case No. 82,339, filed Aug. 9, 2000. The device, which by eliminating the microwave mixers of the non-all optical prior art systems provides advantages over the prior art devices, is shown in FIG. 3. Light from a fiber coupled laser, e.g. at a wavelength of approximately 1550 nm, is amplified and then divided into two paths by a polarization maintaining coupler. In one path, light is amplitude modulated by optical modulator MZM1 driven by a local oscillator LO1. The amplitude modulation causes light to be shifted from the fundamental beam into the RF sidebands. The second path includes two cascaded optical modulators, MZM2 and MZM3, where MZM2 modulates the light at a first frequency, e.g. 18 Ghz, after which the light is amplified by an erbium doped fiber amplifier and then modulated a second time by MZM3 driven by RF In, producing wavelengths at an input fundamental frequency, e.g. 1550 nm, a first modulated fundamental frequency, e.g. xc2x118 Ghz, a fundamental xc2x1RF In, and a second modulated fundamental frequency, e.g. xc2x118 Ghzxc2x1RF In. The light is amplified again before passing through a filter that selects one of the optical sidebands; optionally, the amplified light may be passed through multiple optical filters (not shown). Upon recombining the two paths, the beat signal produced by heterodyning the light at the filtered signal with the sideband produced by LO1 is detected and an intermediate frequency (IF) output is applied to an optical transmission line and then sent to user electronics (not shown). Although the system is advantageous for remote applications since it is composed of two fiber optic links, there may exist some inherent conversion loss (xe2x80x9cCLxe2x80x9d), e.g. on the order of up to about 60 dB, that may be undesirable in some applications. A preamplifier may then be necessitated to compensate for the CL, increasing the system""s cost and complexity, and the system might also exhibit a limited spur free dynamic range.
The object of this invention is to is to provide an apparatus for the improvement of image rejection in image rejection mixing systems while removing harmonic spurs.
Another object of this invention is to provide a device having sufficiently large image rejection capability so as to allow precise determination of frequency for direction finding applications.
These and other objectives are accomplished by the all-optical image reject down converter which maps received radio frequency (RF) into an arbitrary intermediate frequency (IF) range, while precluding interference between the received signals. The invention converts a received radio frequency signal into an arbitrary intermediate frequency for use by an electronic circuit in other devices. An example of the system""s ability to convert a received 9 GHz signal into a 2 GHz intermediate frequency is as follows. Optical light originating from a laser is divided into two paths. Light in a first path is transferred into an optical sideband by a first optical modulator or phase modulator (LO1=7 GHz). Light in a second path is converted into 9 GHz sidebands by a second, single (i.e., non-cascaded) optical modulator, generating light with an optical spectrum that includes many wavelengths. The light from the second optical modulator is then passed through a narrow, tunable optical filter that selects the 9 Ghz sideband, thereby producing an optical spectrum that primarily includes light at a frequency equal to the initial input laser frequency plus the 9 GHz signal. The filtered second path sideband is then heterodyned with the light from the first path, resulting in a down-conversion to 2 Ghz.
The use of a narrow-band optical filter allows the system to select a particular sideband for use in the heterodyne down-conversion. As a result, image frequencies present at the RF input are filtered out and are rejected. The image rejection of the system is a function of filter extinction.
The down-convertor allows telecommunications systems to down-convert densely multiplexed ultra-wide bandwidth channels into low frequency bands where conventional electronics can perform signal processing functions. It provides image reduction to provide unambiguous signals for applications such as direction finding. It exhibits a bandwidth sufficient for permitting multi-octave microwave frequency reception and compression. It has at least an order of magnitude reduction in conversion loss compared to other down-converters. The all optical down-converter is intrinsically remotable and therefore suitable for use in numerous fiber optic and antenna systems.